The present invention relates to fluorinated copolymers with reactive unsaturated side groups and a process for their preparation.
Fluorinated polymers are always employed in industry if particular properties are required, such as low surface tension, high resistance to chemicals or extreme requirements in respect of stability to ageing. For many intended uses, the fluorinated polymers are crosslinked during or after shaping, such as, for example, when used in the rubber field. In the case of non-modified fluorinated elastomers, that is to say which contain no particular reactive sites, these crosslinking reactions require drastic conditions, which means that the crosslinking is difficult to control and the fluorinated polymer may become damaged. To avoid this, reactive sites, so-called cure sites, at which crosslinking can be carried out selectively and under milder conditions are introduced into the polymer chains. Such cure sites are, for example, bromine substituents or iodine substituents, which are introduced into the fluorinated polymer either by copolymerising small amounts of bromine- and/or iodine-containing vinyl compounds with the fluorinated monomers (U.S. Pat. Nos. 3,351,619; 4,035,565; 4,214,060; German Offenlegungsschrift 3,715,210), or by carrying out the polymerisation in the presence of saturated compounds containing iodine or bromine and iodine (German Offenlegungsschrift 2,815,187; German Offenlegungsschrift 3,710,818). Such fluorinated polymers modified by bromine and/or iodine can be crosslinked by free radicals, for example by peroxidic compounds, in the presence of a co-crosslinking agent. Triallyl cyanurate or triallyl isocyanurate can be employed as the co-crosslinking agent (Rubber Chem. Technol. 55 (1982), 1004).
In comparison with, for example, types crosslinked with bisphenols, such fluorinated rubbers crosslinked by peroxides have a considerably improved resistance to bases, nucleophiles and oxidative attack (Kautschuk and Gummi, Kunststoffe 38 (1985), 471).
However, the fluorinated bromine- or iodine-containing polymers have the disadvantage that low molecular weight alkyl bromides or iodides which evaporate out during the crosslinking reaction and in this way impair the processing and use properties may be formed during the free radical crosslinking reaction.
There was therefore a need for fluorinated polymers which can be crosslinked by free radicals, for example by peroxide, under mild reaction conditions and which do not have the abovementioned weaknesses. These properties required could in principle be achieved by the incorporation of polyfunctional monomers, that is to say monomers with more than one reactive double bond, if controlled incorporation maintaining some of the double bond were possible.
However, if fluorinated monomers are polymerised according to the prior art in the presence of polyunsaturated monomers, such as perfluorodivinyl ether (U.S. Pat. No. 3,851,018), considerable crosslinking of the polymers already takes place during the copolymerisation in a manner which is undesirable for many purposes.
A particularly high degree of crosslinking in fluorinated polymers is even achieved by copolymerisation with hexafluorotriallyl isocyanurate (U.S. Pat. No. 4,320,216). Non-crosslinked fluorinated polymers with C--C double bonds in side chains are not accessible by known processes.